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In the field of medicine, radiation may be used for diagnostic, therapeutic and palliative purposes. Therapeutic use of radiation such as x-rays and xcex3-rays typically involves using these rays to eradicate malignant cells. Conventional radiation treatment systems used for medical treatment, such as linear accelerators that produce high-energy x-rays, utilize a remote radiation source external to the targeted tissue. A beam of radiation is directed at the target area, for example a malignant tumor inside the body of a patient. The x-rays penetrate the patient""s body tissue and deliver radiation to the cancer cells, usually seated deep inside the body. This type of treatment is referred to as teletherapy because the radiation source is located at some distance from the target. This treatment suffers from the disadvantage that tissue disposed between the radiation source and the target is exposed to radiation. To reach the cancer cells, the x-rays from an external radiation source must usually penetrate through normal surrounding tissues. Non-cancerous tissues and organs are thus also damaged by the penetrating x-ray radiation.
Brachytherapy, on the other hand, is a form of treatment in which the source of radiation is located close to, or in some cases within, the area receiving treatment. Brachytherapy, a word derived from the ancient Greek word for close (xe2x80x9cbrachyxe2x80x9d), offers a significant advantage over teletherapy, because the radiation is applied primarily to treat only a predefined tissue volume, without significantly affecting the tissue adjacent to the treated volume. The term brachytherapy is commonly used to describe the use of xe2x80x9cseeds,xe2x80x9d i.e. encapsulated radioactive isotopes, which can be placed directly within or adjacent to the target tissue being treated. Handling and disposal of such radioisotopes, however, may impose considerable hazards to both the handling personnel and the environment. Also, introduction of the radioisotopes requires invasive procedures which have potential side-effects, such as the possibility of infection. Moreover, there is no ability to provide selective control of time dosage or radiation intensity.
The term xe2x80x9cx-ray brachytherapyxe2x80x9d is defined for purposes of this application as x-ray radiation treatment in which the x-ray source is located close to or within the area receiving treatment. An x-ray brachytherapy system, which utilizes a miniaturized low power radiation source that can be inserted into, and activated from within, a patient""s body, is disclosed in U.S. Pat. No. 5,153,900 issued to Nomikos et al., U.S. Pat. No. 5,369,679 to Sliski et al., U.S. Pat. No. 5,422,926 to Smith et al., and U.S. Pat. No. 5,428,658 to Oettinger et al., all owned by the assignee of the present application, all of which are hereby incorporated by reference.
The x-ray brachytherapy systems disclosed in the above-referenced patents include miniaturized, insertable x-ray probes that are capable of controllably producing and delivering low power x-ray radiation, while positioned within or in proximity to a predetermined region to be irradiated. In this way, x-ray radiation need not pass through the patient""s skin, bone, or other tissue prior to reaching the target tissue. The probe may be fully or partially implanted into, or surface-mounted onto a desired area within a treatment region of a patient. X-rays are emitted from a nominal, or effective xe2x80x9cpointxe2x80x9d source located within or adjacent to the desired region to be irradiated, so that substantially only the desired region is irradiated, while irradiation of other regions are minimized. X-ray brachytherapy offers the advantages of brachytherapy, while avoiding the use and handling of radioisotopes. Also, x-ray brachytherapy allows the operator to control over time the dosage of the delivered x-ray radiation.
X-ray brachytherapy treatment generally involves positioning the insertable probe into or adjacent to the tumor or the site where the tumor or a portion of the tumor was removed to treat the tissue adjacent the site with a local boost of radiation. X-ray probes of the type generally disclosed in U.S. Pat. No. 5,153,900 include a capsule, and a hollow, tubular probe or catheter extending from the capsule along an axis, and having an x-ray emitting target element at its distal end. The probe may enclose an electron source, such as a thermionic cathode. In one form of a thermionic cathode, a filament is resistively heated with a current. This in turn heats the cathode so that electrons are generated by thermionic emission.
In another form of an x-ray brachytherapy device, as disclosed in U.S. Pat. No. 5,428,658, an x-ray probe may include a flexible probe, such as a flexible fiber optic cable enclosed within a metallic sheath. The x-ray probe may also include a substantially rigid, evacuated capsule that is coupled to a distal end of the flexible probe. The capsule encloses an optically activated electron source, such as a photocathode, and an x-ray emissive target element. In a photocathode configuration, a photoemissive substance is irradiated by a LED or a laser source, causing the generation of free electrons. Typically, a flexible fiber optic cable couples light from a laser source or a.LED to the photocathode.
U.S. patent application Ser. No. 09/884,561 (hereinafter the xe2x80x9c""561 applicationxe2x80x9d) (commonly owned by the assignee of the present invention and incorporated herein by reference), entitled xe2x80x9cOptically Driven Therapeutic Radiation Source,xe2x80x9d discloses an optically driven (for example, laser driven) therapeutic radiation source that uses a reduced-power, increased efficiency electron source to generate electrons with minimal heat loss. The ""561 application discloses the use of laser energy to heat an electron emissive surface of a thermionic emitter, instead of using an electric current to ohmically heat an electron emissive surface of a thermionic emitter. With the optically driven thermionic emitter, electrons can be produced in a quantity sufficient to produce the electron current necessary for generating therapeutic radiation at the target, while significantly reducing the requisite power requirements. U.S. patent application Ser. No. 10/005,290 hereby discloses a therapeutic radiation source having an in situ radiation detector, which permits real-time monitoring of the therapeutic radiation that is being generated and delivered.
Even though the above-discussed miniature radiation sources can generate x-rays local to the target tissue, it is difficult to provide a uniform, or other desired, dose of radiation to an irregularly shaped target tissue, using these radiation sources. These miniature radiation sources generally act as point sources of therapeutic radiation. The intensity of the radiation from a point source decreases uniformly with approximately the square of the distance (R) from the source (i.e., 1/R2). Since body cavities, or the beds of resected tumors, are not generally spherically symmetrical, a point source within a body cavity or central to the resected tumor bed will not deliver a uniform dose of radiation to the tissue lining of the cavity or bed. Likewise, a point source at the center of a non-spherical tumor will not deliver radiation with an isodose contour matching the peripheral surface of the tumor.
The treatment regions within a patient""s anatomical structure are usually not adapted for uniform or spherically isotropic patterns of irradiation, because the organs or body cavities being treated during radiation therapy usually have arbitrary and irregular shapes and geometries. The areas of a patient""s body requiring treatment may be characterized by twists and bends. In some cases, the geometry of the target region may not be fixed, as in the bladder for example, which has a flexible inner wall without a well-defined shape. Also, some treatment procedures may require delivery of localized radiation to portions of the human body that are not easily accessible. Cancerous tumors are usually shaped irregularly, and are distributed randomly across a given anatomical region. A single point source of therapeutic radiation, even when inserted into and activated within a patient""s body, cannot deliver a uniform dose of radiation to a desired area within an irregularly shaped body cavity or organ, nor can it deliver more complex radiation dose patterns that may be desirable or required for some cases. Similarly, a single point source at the center of a non-spherical tumor will not deliver radiation with an isodose contour matching the peripheral surface of the tumor, as discussed earlier.
For the foregoing reasons, there is a need for devices and methods which overcome the above-discussed limitations of brachytherapy, by enabling a more versatile, efficient, and versatile delivery of localized therapeutic radiation, while still preserving the advantages of brachytherapy. In particular, an arrangement in which a plurality of point-like sources of therapeutic radiation are positioned over the desired treatment region as a one- or a multi-dimensional array would significantly increase user control over the intensity and duration of the therapeutic radiation being delivered, and would enable the user to achieve complex radiation profiles.
The present invention provides a system for delivering therapeutic radiation, in which a plurality of therapeutic radiation sources are arranged over a desired treatment region as a one- or a multi-dimensional array. In one embodiment, the therapeutic radiation consists of x-rays, although the scope of this invention is not limited to x-ray sources. In a preferred embodiment of the invention, the plurality of therapeutic radiation sources are selectively and moveably disposed on a two-dimensional (2-D) surface, and arranged into a two-dimensional array. Alternatively, the plurality of therapeutic radiation sources may be selectively and moveably disposed along an axis so as to form a one-dimensional (1-D) array. Alternatively, the plurality of therapeutic radiation sources may be selectively and moveably disposed within a three-dimensional (3-D) volume and arranged into a three-dimensional array. The therapeutic radiation sources may be disposed on two-dimensional surfaces having any desired configuration, including rigid, flexible, planar, concave, convex, spherical, or cylindrical surfaces. The axes defining the arrays may also have any desired shape or configuration, including straight, curvilinear, rigid, or flexible axes. The therapeutic radiation sources may be regularly or variably spaced along the one- or multi-dimensional arrays.
Each therapeutic radiation source includes an electron source, such as a cathode. The cathode may be a thermionic cathode, a cold cathode or a photocathode. The thermionic cathode may be a resistively heated thermionic cathode, or a laser-heated thermionic cathode. The electron source emits electrons to generate an electron beam along a beam path. A target element is positioned in the beam path. The target element includes at least one radiation emissive material for emitting radiation, for example x-rays, in response to incident accelerated electrons from the electron beam.
An accelerating voltage is provided between each electron source and each associated target element, so that an accelerating electric field is established which acts to accelerate electrons emitted from the electron source toward the associated target element. The therapeutic radiation sources are individually controllable, i.e. the intensity and duration of the emitted therapeutic radiation can be individually controlled for each therapeutic radiation source.
In an embodiment of the invention, the system for delivering therapeutic radiation further includes a plurality of optical delivery structures, each optical delivery structure being associated with a corresponding one of the plurality of therapeutic radiation sources. In a preferred embodiment, the optical delivery structure is a fiber optic cable. The system further includes one or more light sources which generate a beam of light directed to the proximal end of each fiber optic cable. Preferably, the one or more light sources are laser sources that generate a laser beam. In this embodiment, the electron source in each therapeutic radiation source emits electrons in response to light transmitted to the distal end of the associated fiber optic cable. The electron source may be an optically heated thermionic cathode, or a photocathode. The optically heated thermionic cathode has an electron emissive surface adapted to emit electrons when heated to a sufficient temperature by a beam of optical radiation, such as laser light. The photocathode has a photoemissive surface, and is responsive to optical radiation incident thereon to emit electrons from the photoemissive surface.